On 2015-01-28 00:57, Jean Lopes wrote:
I'm not really good at math, maybe I am missing something obvious ? Maybe some pointers as of where to start studying math in order to avoid this brute force attempts, at least to help in this particular problem
I'm not too familiar with 'Project Euler', but summing all multiples of 3 and 5 below some given 'n' made me think: e.g. 16 it would be... 3 + 5 + 6 + 9 + 10 + 12 + 15 = 3 + 6 + 9 + 12 + 15 + 5 + 10 | reordered summands = 3 + 6 + 9 + 12 + 15 + 5 + 10 + 15 - 15 | +15-15 appended to prepare factoring out = 3 + 6 + 9 + 12 + 15 + 5 + 10 + 15 - 15 | whitespace for readability = 3 * (1+2+3+4+5) + 5 * (1+2+3) - 15 * (1) | factoring out Now I remembered a trick to get the sum of the first N numbers (I only remember it's called "der kleine Gauß" in german): 1 + 2 + ... + n = (n^2 + n) / 2 Maybe that'll help. - Frerich